Packing the car for Huzzah! reminded me that I would like to set up a couple of other easy to transport "game in a box" projects, suitable to take along on a trip by plane, or to tuck into some underutilized bit of cubic in the car on the way to a convention. This time around we took the "Big Box of Six" as a contingency item, and ended up improvising a Basic Impetus game. That would have looked a little nicer if we'd had a larger cloth, but the BBoS is usually used for DBA or Hordes of the Things, so a 2' by 2' (or 60cm square for the rest of the world) cloth is all that we usually need. The box is not yet full, so there is room for expansion.
The ideal game in a box would remain self-contained, so that it doesn't have to be reassembled from scattered pieces just before a convention, and occupy 2-6 players for 1-3 hrs to fill odd time gaps. An ambitious one would also be deployable as a convention game, which I'd call 6 players for 3-4 hours. In addition to the BBoS, I built a box with two 6mm Spanish Civil War intro army packs from Irregular Miniatures back in the mid-90s. When it was originally completed, each army had about five battalions of troops plus some support elements, as based for Command Decision. (2nd edition at the time...) That would provide a fairly satisfactory game for 6, perhaps just a little light, and a solid game for 2-4 players. I've also deployed it more recently for playtests with newer rules.
We've tossed around the idea of housing a game in a plastic shoebox. I've been considering ordering something like the Pegasus 1/32 scale gladiators, for which a shoebox should be plenty. Putting a 20mm battle game in a volume that size would be an interesting challenge...